Circuit breakers are used in electrical distribution systems to protect electrical circuits from abnormally high currents caused by faults and overloads. When a circuit breaker detects a fault or unacceptably high and prolonged overload it responds by opening or “breaking” its circuit to prevent overcurrents from flowing in the circuit, which otherwise could damage electrical equipment or cause fire due to excessive heating of the circuit's electrical wiring.
A circuit breaker is essentially a mechanically operated electrical switch with contacts that remain closed during normal operating conditions and that are opened when a fault or prolonged overload is detected. FIG. 1 is simplified drawing showing the salient components of a typical circuit breaker 100, configured in both the closed (“Not Tripped”) and open (“Tripped”) positions. The circuit breaker 100 includes LINE-IN and LINE-OUT terminals, which connect to an electrical power source (often from a power distribution panel or “panelbox”) and a load (not shown in the drawing); a solenoid 102; a trip bar 104; a flexible bimetallic strip 106; a latch 108; and contacts 110. During normal operating conditions, when the contacts 110 are closed (“Not Tripped”), line current flows into the LINE-IN terminal, through the solenoid 102 coil and flexible bimetallic strip 106, and finally into the load, via the LINE-OUT terminal. There are two ways in which the circuit breaker 102 can be caused to trip—one due to a prolonged and unacceptably high overload condition and the other due to a short circuit. During an overload, when the line current is high but not at a level indicative of a short circuit, the current heats the bimetallic strip 106, causing it to bend and deflect toward the upper lever of the trip bar 104. The higher the current is the more the bimetallic strip 106 deflects. If the magnitude of the current is abnormally high and sustained over a period of time, the bimetallic strip 106 will deflect far enough to push against the upper lever of the trip bar 104 and cause the opposing end of the trip bar 104 to disengage from the latch 108. When the trip bar 104 disengages from the latch 108, the contacts 110 are then allowed to open (“Tripped”), as shown in the right-hand portion of FIG. 1. The contacts 110 of the circuit breaker 100 will also open when a short circuit occurs (the second way the circuit breaker can be tripped), and more rapidly. Specifically, when a short circuit occurs the solenoid 102 reacts by pulling its plunger inside the solenoid coil. As the plunger is retracted to inside the coil it also engages and pulls the trip bar 104, causing the trip bar 104 to rotate about its axis, disengage from the latch 108, and allow the contacts 110 to open.
Although mechanical circuit breakers have been in widespread use for many years, they have a number of important limitations. One limitation relates to the fact that a person must be physically present to reset the circuit breaker 100 after a fault or overload has been cleared. A person resets the circuit breaker 100 by manipulating a reset lever (not shown in FIG. 1) built into the circuit breaker. When the person “flips” the reset lever (similar to how a light switch is flipped) the trip bar 104, latch 108, and contacts 110 are reset to their “Not Tripped” positions. Some larger circuit breakers include electric motors that are adapted to reset the circuit breaker's contacts, thus obviating the need for a person to be present. However, motors are not always reliable, so relying on them to reset circuit breakers is not an optimal solution.
Mechanical circuit breakers are also limited in their ability to react quickly to faults, typically requiring a few or tens of milliseconds to detect and fully isolate faults. The slow reaction time is undesirable since it increases the risk that electrical equipment might be damaged and the possibility of fire.
Due to their mechanical construction and operation, the current and time thresholds at which mechanical circuit breakers trip can also vary considerably among circuit breakers of the same type and rating, even for circuit breakers of the same type and rating provided by the same manufacturer. This variability is typically shown in the time-current characteristic curves (i.e., “tripping curves”) provided by the manufacturer, similar to as shown in FIG. 2. The upper left portion of the tripping curve represents the circuit breaker's thermal response to overloads (which, as explained above, the circuit breaker responds to using the bimetallic strip 106), and the lower right portion of the tripping curves provides information concerning how fast the circuit breaker can respond to short circuit faults. Since the performance of the circuit breaker will vary from one part to another, the tripping curve is expressed as a tolerance “band,” rather than as a line. The tripping curves depicted in FIG. 2 show that the circuit breaker of this particular type and rating is guaranteed to clear a short circuit in no longer than 20 milliseconds. They also show that the circuit breaker will tolerate overload currents (i.e., “overcurrents”) of up to five times (×5) the circuit breaker's rated current for up to one second. Actually, the tolerance band reveals that some circuit breakers of this same type and rating might be able to tolerate a current of ×5 the rated current for longer than one second. However, due to part-to-part variations, the manufacturer can only guarantee that any given circuit breaker will tolerate a current of ×5 the rated current for a maximum of one second.
Another limitation associated with mechanical circuit breakers is that electrical arcs are produced across the air gap that forms between the contacts when high voltages are interrupted. Electrical arcing is highly undesirable since not only can it cause pitting of the circuit breaker's contacts but because it can cause fires or explosions in environments that contain flammable vapors, for example as may occur in a room or area that is not well ventilated, such as a room or area that is not well ventilated and which contains hydrogen-producing batteries. Conventional mechanical circuit breakers often include some mechanism to extinguish arcs as they are produced, such as arc chutes that divide and cool the arcs, jet chambers that blast vaporized oil through the arcs, or compressed air to blow out the arcs. However, none of these techniques prevent arcing from occurring in the first place.
Finally, and again due to their mechanical construction and operation, the lifespans of mechanical circuit breakers are limited and shorter than desired. Not only does arc-induced pitting wear out the circuit breaker's contacts 110 over time, the other components of the circuit breaker 100 that must be mechanically manipulated to open and close the contacts 110 (e.g., trip bar 104, latch 108, springs, solenoid 102, etc.) are subjected to wear and tear every time the breaker is opened and closed and thus are also prone to failure.
Considering the various limitations of conventional mechanical circuit breakers, it would be desirable to have a circuit protection device and associated method that is more precise than conventional mechanical circuit breakers; that can detect and respond to faults much more rapidly than conventional mechanical circuit breakers; that prevents electrical arcing from occurring when high voltages are interrupted; that can be reset without a person being present; that is not subject to the wear and tear that besets conventional mechanical circuit breakers; and that has a lifespan much longer than conventional mechanical circuit breakers.